Fuel cells can be used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied as the anode reactant to the anode of the fuel cell and oxygen, or air, is supplied as the cathode reactant to the cathode of the fuel cell. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of non-porous, electrically-conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged electrically in series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. As such, these MEAs are relatively expensive to manufacture and require certain conditions, including proper water management and humidification and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
The electrically conductive plates sandwiching the MEAs may contain a reactant flow field for distributing the fuel cell's gaseous reactants over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels. In the fuel cell stack, a plurality of cells are stacked together electrically in series while being separated one from the next by a gas impermeable, electrically conductive bipolar plate. Water (also known as product water) is generated at the cathode electrode based on the electrochemical reactions between hydrogen and oxygen occurring within the MEA. Efficient operation of a fuel cell depends on the ability to provide proper and effective water management in the system.
The active area of the fuel cell(s) and flow fields are sized for the maximum power output of the fuel cell stack. During full or high power operation, the cathode reactant flow velocity is sufficient to transport liquid water from the flow field. During times of reduced power output, however, the quantity (mass flow rate) of anode and cathode reactants flowing into the active area and associated flow fields is reduced and the resulting reduced flow velocity may not be sufficient to transport the liquid water from the active area and flow fields. To compensate, the quantity of cathode reactant flowing into the flow fields could be increased beyond that required in order to provide the velocity necessary to remove the water, however, this would require a significant excess quantity of cathode reactant. Supplying significant excess cathode reactant consumes energy and decreases the fuel cell system efficiency, and may also have an adverse effect on the humidification and operation of the MEA. Therefore, it would be advantageous to maintain the cathode reactant flow velocity at a rate that transports liquid water from the active area and flow fields while minimizing and/or eliminating excessive cathode reactant flow.